This invention relates generally to gas generant materials such as used to inflate automotive inflatable restraint airbag cushions and, more particularly, to gas generant materials having specific geometric forms or shapes.
Gas generating materials are useful in a variety of different contexts. One significant use for such compositions is in the operation of automotive inflatable restraint airbag cushions. It is well known to protect a vehicle occupant using a cushion or bag, e.g., an xe2x80x9cairbag cushion,xe2x80x9d that is inflated or expanded with gas when the vehicle encounters sudden deceleration, such as in the event of a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Such systems typically also include one or more crash sensors mounted on or to the frame or body of the vehicle to detect sudden decelerations of the vehicle and to electronically trigger activation of the system. Upon actuation of the system, the cushion begins to be inflated in a matter of no more than a few milliseconds with gas produced or supplied by a device commonly referred to as an xe2x80x9cinflator.xe2x80x9d In practice, such an airbag cushion is desirably deployed into a location within the vehicle between the occupant and certain parts of the vehicle interior, such as a door, steering wheel, instrument panel or the like, to prevent or avoid the occupant from forcibly striking such part(s) of the vehicle interior.
Various types of inflator devices have been disclosed in the art for the inflation of an airbag such as used in inflatable restraint systems. One type of known inflator device derives inflation gas from a combustible pyrotechnic gas generating material which, upon ignition, generates a quantity of gas sufficient to inflate the airbag.
Gas generant materials having various forms have been developed or proposed for use in such safety system applications. For example, inflators which contain gas generant in the form of cylindrical pressed pellets or tablets having a generally circular or rounded cross section and which cylindrical tablets can be randomly packed into a combustion chamber of the inflator device are well known in the art. Alternatively, the art has also described the use of perforated or annular-shaped cylinders of pyrotechnic compositions, see EP 867 347, for example.
In practice, an inflator device is typically selected for a particular installation based at least in part on the performance capabilities required of the inflator in such installation. xe2x80x9cRise ratexe2x80x9d, i.e., the rate at which the gas output from an inflator increases pressure, as measured when such gas output is directed into a closed volume, is a common performance parameter used in the design, selection and evaluation of inflator devices for particular restraint system installations.
Relatively aggressive requirements for inflator performance have been established by some automobile companies for at least certain applications. For example, assuming that an occupant is properly seated and wearing an appropriate seat belt, such automobile companies may seek for the onset of airbag cushion deployment to occur at relatively higher minimum speeds such that less time may be required for the airbag cushion to properly deploy. For instance, relatively aggressive requirements for an inflator for an airbag cushion to provide front impact protection to a driver (i.e., a driver inflator) may include the following (as measured in a 60-liter tank): maximum pressure (Pmax) as 230 kPa; pressure at 20 ms following initiation (P20) as 145 kPa; and rise rate, here specifically defined in terms of the maximum rise in pressure over a 5-ms time period, as 56 kPa. In contrast, the requirements for a typical xe2x80x9cdepoweredxe2x80x9d inflator for current systems are: Pmax of 160 kPa to 180 kPa; P20 of 84 kPa; and a maximum rise rate over a 5-ms time period of 40 kPa.
In general, the rise rate of a pyrotechnic-based inflator device as described above is primarily a factor of the burn rate and the surface area of the pyrotechnic gas generant material. Pyrotechnic gas generant materials having different burn rate characteristics have been developed. For example, U.S. Pat. No. 5,608,183, whose disclosure is hereby incorporated by reference herein and made a part hereof, discloses one such family of gas generant material. Such gas generant materials have various desirable properties or characteristics including: high gas yield, excellent aging and produce or result in desirably low levels of trace gas products such as nitric oxide and carbon monoxide. Considerable experimentation with this particular family of pyrotechnics has shown that these pyrotechnic materials generally have a burn rate range of 0.5 ips to 0.6 ips at 1000 psi and that there is scant likelihood that the burn rate can be significantly increased beyond this range.
Efforts to increase the surface area of such and similar gas generant materials are generally subject to several limitations. For example, standard cylindrical gas generant material tablets have a diameter of 0.25 inches. The thickness of such tablets can be no less than about 0.055 inches because of safety issues for manufacturing. In particular, the pressing of tablets of such materials in a thickness of significantly less than 0.055 inches may result in unacceptable risks of inadvertent ignition or reaction during the manufacturing or production process.
Alternatively, the surface area of gas generant material can be increased by increasing the load of tablets having such dimensions. The use of an increased load of such dimensioned tablets, however, is generally limited as most inflators are volume constrained and there is no xe2x80x9cextraxe2x80x9d volume to contain the increased load of gas generant material. Further, increasing the gas generant material load in an inflator may require that the inflator be specially constructed (e.g., have thicker walls) in order to properly ensure that the maximum combustion pressure that the inflator can tolerate will not be exceeded.
Thus, there is a need and a demand for a tablet form of gas generant which will yield a higher surface area at a constant gas generant load while satisfying minimum tablet thickness requirements such as necessitated by manufacturing safety issues.
A general object of the invention is to provide an improved form of gas generant.
A more specific objective of the invention is to overcome one or more of the problems described above.
The general object of the invention can be attained, at least in part and in accordance with one preferred embodiment of the invention, through a body of gas generant material in the form of a tablet having a triangular cross section and with a top face, a bottom face and a side face extending between the top and bottom surface.
The prior art generally fails to provide a tablet form of gas generant which will yield a higher surface area at a constant gas generant load while satisfying minimum tablet thickness requirements such as necessitated by manufacturing safety issues. In particular, the prior art generally fails to provide a tablet form of gas generant material such will more readily permit the use of low to moderate burn rate gas generant materials in applications having relatively high minimum rise rate requirements.
The invention further comprehends, in accordance with another embodiment of the invention, a body of gas generant material having a burn rate of no more than about 0.6 ips at 1000 psi and of a form having a cup-shaped top face, a cup-shaped bottom face and a side face extending between the top and bottom surface. The side face is composed of three arced side face segments. Each one of the three side face segments is joined with each of the other two of the three side face segments via a rounded corner.
The invention still further comprehends a method of generating gas. In accordance with one preferred embodiment of the invention such method involves igniting a plurality of specially shaped gas generant material bodies disposed within a combustion chamber. In particular, the specially shaped gas generant material bodies are in the form of a tablet having a triangular cross section and with a top face, a bottom face and a side face extending between the top and bottom surface.
Other objects and advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawings.